Heat desensitizing of convertible plate

ABSTRACT

A CONVERTIBLE PLATE ADAPTED FOR CONVERSION BY ELECTROSTATIC PROCEDURES INTO A LITHOGRAPHIC PRINTING PLATE, COMPRISING A SHEET BASE FORMED FROM A PAPER WEB, AND A DIELECTRIC FILM EXTENDING OVER THE SHEET BASE ADAPTED TO RECEIVE AN IMAGE-DEFINING ELECTROSTATIC CHARGE. THE FILM IS DEGRADABLE BY THE APPLICATION OF HEAT. IN PREPARING A PRINTING PLATE FROM THE CONVERTIBLE PLATE, THE IMAGE-DEFINING ELECTROSTATIC CHARGE IS DEVELOPED, AND IN NONIMAGE AREAS THE FILM IS DENSITIZED BYU DEGRADING THE FILM WITH HEAT. THIS RENDERS SUCH NONIMAGE AREAS OF THE FILM HYDROPHILIC, OR WETTABLE WITH AN AQUEOUS FOUNTAIN SOLUTION, IN A LITHOGRAPHIC PRINTING PROCESS.

United States Patent Int. Cl. G03g US. Cl. 96-1 4 Claims ABSTRACT OF THEDISCLOSURE A convertible plate adapted for conversion by electrostaticprocedures into a lithographic printing plate, comprising a sheet baseformed from a paper web, and a dielectric film extending over the sheetbase adapted to receive an image-defining electrostatic charge. The filmis degradable by the application of heat. In preparing a printing platefrom the convertible plate, the image-defining electrostatic charge isdeveloped, and in nonimage areas the film is densitized by degrading thefilm with heat. This renders such nonimage areas of the filmhydrophilic, or wettable with an aqueous fountain solution, in alithographic printing process.

This invention relates generally to the preparation of a lithographicprinting plate using electrostatic procedures. It more particularlyconcerns the preparation of such a printing plate where properhydrophilic properties in nonimage areas of the plate, after developermaterial has been applied to the plate, are obtained by heating thedielectric film in the plate extending over such areas whereby it breaksdown to be rendered hydrophilic.

As is well known in the art, in lithographic printing a plate isprepared with image areas on the surface of the plate renderedoleophilic (such generally being hydrophobic), and with nonimage areashydrophilic. Such a plate may then be wet with an aqueous fountainsolution, which functions to wet the hydrophilic nonimage areas to theexclusion of the image areas. When such a plate is then contacted withan ink roll coated with an oil-base ink, the ink deposits on theoleophilic areas where the image appears exclusively of the other areas,with such ink then being transferable to a paper sheet to produce afinal print.

Several procedures have been proposed for preparing an image on a plateto be used in lithographic printing, using electrostatic printingprinciples. In one type of procedure, referred to herein aselectrophotographic imaging, because of its resemblance to anelectrophotographic printing process, a sheet including aphotoconductive layer is first given a uniform charge over its entireface, in the dark. Then an image-defining electrostatic charge may beformed by exposure of the charged photoconductive layer to light,through a positive image, with the charge being removed in those areasreceiving light, and remaining where such is not received, much like aphotographic process. In another type of electrostatic process, referredto herein as electrographic imaging because of its resemblance toelectrographic printing, an image-defining electrostatic charge is laiddirectly down upon an insulating or dielectric surface in a sheet, usingsuch electronic devices as a cathode ray tube, a pin matrix and pulsingcorona discharge, or by means of a direct image transfer from onesurface to another. With either of such electrostatic imagingprocedures, after the image-defining charge has been formed on thesheet, the image charge area may be developed using a finely divideddeveloper material, which deposits on the sheet where the chargeappears. This developer material is fixed by heating, and is oleophilic.The sheet or plate with the image developed thereon may be employed asthe oifset master in litho- 3,573,040 Patented Mar. 30, 1971 graphicprinting, providing the sheet either after development or after furtherprocessing has proper hydrophilic, oleophilic balance, with areas of thedeveloped image being hydrophobic or oleophilic, and nonimage areasbeing hydrophilic.

In a more particular sense, the invention concerns the preparation of alithographic printing plate using an electrographic imaging procedure,where the image defining charge is prepared by laying a charge directlydown on an insulating or dielectric surface in the plate. Withelectrographic imaging, the dielectric film must have high dielectricproperties, the film ordinarily being somewhat thinner than thephotoconductive film found in plates to be used in electrophotographicimaging, and it is with such highly dielectric films that the inventionis particularly applicable.

A problem arises in the selection of the resins used to prepare thedielectric film mentioned, since most, if not all, known resins whichare highly dielectric in nature are also hydrophobic, and thus notwettable with an aqueous fountain solution. Desensitizing is thusrequired, whereby the hydrophilic property desired in the nonimage areasis obtained.

Generally, this invention features a convertible plate, adapted forconversion into a printing plate by an electrographic printing process,where the outer dielectric film, normally hydrophobic in nature, isdesensitized in nonimage areas into a film having hydrophilic propertiesby the application of heat. The heat degrades the film in nonimageareas, to produce the result desired.

An important object of the invention also is to provide a novel processfor preparing a lithographic offset master, which features the step ofheating nonimage areas in the dielectric film in the plate, to changethe characteristic of such nonimage areas from one of which is generallyhydrophobic in nature to one which is suitably hydrophilic.

Thus, and considering a specific embodiment of the invention, a plateadapted for conversion into an olfset master is contemplated thatincludes an underlying conductive layer (to dissipate stray electricalcharges) surfaced by a dielectric film composed of saran and a catalystpromoting heat degradation of the resin. In developing an image on theplate, a developer may be used which is fixed by the application ofheat, :and such heat used in fixing the developer may also be employedin producing desensitizing in nonimage areas through heat degradation ofthe dielectric film.

Various other features and novel objects of the inven tion will becomemore fully apparent, as the following description is read in conjunctionwith certain specified examples, included for the purpose ofillustration.

As contemplated by this invention, a convertible plate adapted forconversion into a lithographic printing plate or offset master may beprepared from a paper sheet or web which provides support for variouscoatings applied thereover. There are no particular requirements forsuch a web, other than it have sufficient strength to withstand theoperations to which the plate is subjected in making from it an offsetmaster, and such other requirements as are dictated by economic reasons,etc. Plates, such as metal plates could be used, although such are moreexpensive, and are not well suited for use in a disposable type ofproduct.

In manufacturing the convertible plate, it is preferred that first aconductive coating of relatively conductive material be prepared overthe paper sheet or web, and over such conductive coating a so-calledholdout coating be applied to form what is referred to herein as a sheetbase. The conductive coating is advantageous, since such accommodatesthe dissipation of stray electrical charges in the plate, making itpossible ultimately to obtain clearer prints with less background, etc.The holdout coating is advantageous, in that such provides a coveringover the conductive coating (the latter ordinarily being quite thinlyspread) which inhibits impregnation of the paper sheet or web by anycoating material which is applied subsequently over the holdout coating.

Optimumly, the conductive coating or layer should have a resistance(surface resistivity) lying somewhere within the range of about 10 to 10ohms per square. With lower resistances in the conductive layer, changeretention problems are introduced, and with greater resistances, strayelectrical charges are not sufliciently Well dissipated to produceoptimum image resolution.

Illustrative of a resinous material useful in the preparation of theconductive coating is a vinylbenzyl quaternary ammonium chloride resin.This is a water soluble resin, with a resistance of about 10 ohms, a 50%humidity and room temperature (or 70 F.). The resin is a good one toemploy, since it is relatively humidity insensitive, meaning that itretains its conductance at low humidity conditions. Another example of amaterial useful for the conductive coating is a saltbridge type ofsolution, prepared from equal parts of lithium chloride and glycerin,and Water. The coating materials are distributed in a suitable liquidvehicle, i.e., dissolved in water in the case of the materialsindicated, to produce the consistency desired for the coating process.Ordinarily relatively thin coatings suffice for the conductive layer,and normally spreads of from 0.5 to 2.0 pounds per ream, on a solidsbasis, are utilized (ream as used herein refers to 3,000 square feet ofpaper).

With respect to the holdout layer or coating, in general the requirementfor the holdout coating is that it resist dissolution by the materials,more particularly the solvent, found in the subsequently applied coatingwhich forms the dielectric film in the completed plate. The holdoutcoating has no greater and usually insignificant conductivity inrelation to the conductivity of the conductive coating. The principalfunction of the holdout layer is to form an impentrable barrierinhibiting impregnation of the paper sheet or web. Illustrative ofcoating compositions suitable for use as the holdout coating is oneprepared from mineral pigment, such as clay, and starch, inapproximately 2 to 1 proportions, dispersed in water to obtain a properconsistency for spreading, with such applied at from 1 to 5 pounds perream, on a solids basis. As another example, such coating may beprepared from approximately the same proportions of clay and polyvinylalcohol, suitably disposed in water to obtain required spreadingcharacteristics. With the dielectric film which is prepared over theholdout coating prepared from compositions comprising saran dissolved inan organic solvent, such as toluene, methylethyl ketone, etc., whenholdout coatings are prepared and dried from the compositions indicated,such form films resistant to dissolution by such organic solvents,whereby their integrity is maintained during the application of thecoating composition which forms the dielectric film.

Considering in more detail the dielectric film and its preparation, thefilm, after being prepared, preferably should have high dielectricproperties, i.e., a resistance (surface resistivity) ranging upwardlyfrom about ohms per square. Such is necessary, to enable any imagedefining charge prepared on the film to remain without dissipation ofthe charge, which would result in unclear prints. Where electrographicimaging procedures are contemplated, optimum imaging results have beenfound to be obtained if the dielectric film is prepared as a coatingwith spreads ranging from about 5 to 9 pounds per ream. With lowercoating weights, marginal density is obtained in any image ultimatelyproduced, and with higher coating weights, print resolution is affected.

According to this invention, the dielectric film may be prepared fromsaran, which may be polyvinylidene chloride, or more usually a copolymerof vinylidene chloride containing minor proportions of vinyl chloride,acrylonitrile, etc. The film also contains a metal element-containingcatalyst, such as a zinc or iron compound, which promotes degradation ofthe saran film upon the application of heat. This enables the film to bedegraded sufficiently in a matter of minutes to become wettable withwater. Also preferably included in the dielectric coating compositionare minor proportions of resins such as a polymethacrylate (included forthe purpose of toughening the ultimate film produced), and aplasticizer, such as a silicone resin. Certain inert materials such assilica, and other mineral pigments, may also be included, to impartnonglossiness to the ultimate product, to inhibit blocking, and toincrease the surface area of the dielectric film, whereby heatdegradation is further promoted.

Describing the preparation of a convertible plate according to theinvention, conventional 38-lb. label base paper was used as the sheetover which various coating compositions were applied to obtain the finalplate.

A conductive coating was prepared over this sheet using a coatingcomposition comprising parts Dow QX 2611.7 (a 40% solids aqueoussolution of vinylbenzyl quaternary ammonium chloride resin), 15 partsborax, and sufiicient additional Water to lower the percent solids ofthe coating composition to 27% by weight. The borax was included to forma gel on the inner face of the holdout coating subsequently applied overthe conductive coating, with such gel inhibiting impregnation of thepaper web by the holdout coating. Direct steam was used to dissolve theborax. The coating composition was spread at 0.75 pound per ream, on asolids basis. The coating formed had a resistance at 50% humidity and 70F., of about 10 ohms per square.

A holdout coating composition was prepared from 100 parts mineralpigment, i.e., filler clay, 50 parts polyvinyl alcohol, and 600 parts ofwater. This coating composition was applied over the conductive coating,after the former had dried, with an air knife coater, and with a spreadof from 2 to 3 pounds per ream, on a solids basis. Preferably arelatively low viscosity polyvinyl alcohol is employed to promotespreading, and the particular polyvinyl alcohol utilized had a viscosityof below 6 centipoise, at 4% solids, at 70 F.

With the conductive and holdout coatings applied, the sheet base formedwas supercalendered, to prepare it for the reception of the dielectricfilm.

A dielectric film coating composition was prepared comprising 60 partsDow Saran F220 (a vinylidene chlorideacrylonitrile copolymer), 20 partspolymethylmethacrylate resin, 20 parts Dow DC 840 silicone resin, 20parts silica aerogel (Monsanto Santocel PRC), 25 parts mineral pigment(Zinc oxide New Jersey Green Seal 8), and 0.072 part FeCl -6H O. Theferric chloride was dissolved in methylethyl ketone solvent before itsaddition to the coating composition. The components of the coatingcomposition were dispersed in 350 parts of a mixture of 50% toluene and50 methylethyl ketone.

In preparing the dielectric film, a spread of the above composition ofabout 8 pounds per ream was utilized. The dielectric film prepared had aresistance of about 10 ohms per square.

A convertible plate prepared as above had an imagedefining chargeprepared thereon by an electrographic imaging process, i.e., by laying acharge directly down upon the dielectric film in the plate. This wasthen developed by sprinkling toner or developer powder over the platewith such powder being attracted to the image defining charge on theplate. The developer powder was conventional, and possessed oleophilicproperties and was attractive to greasy inks. Positive toners useful inthe development of such plates are discussed in US. Pat. 3,107,169, toBornarth, entitled, The Process of Producing Lithographic ElectrostaticPrinting Plates.

With the toner distributed, such was then fixed to the dielectric filmby placing the plate in an oven and heating the plate to cure the resin.Heating was continued for about one and one-half minutes, at about 350F. The heating not only fixed the resin toner to produce areas ofdeveloped image, but also was effective to desensitize the dielectricfilm in nonimage areas, whereby the film in such areas becamehydrophilic and easily wet with water. The degradation of the resinproduced by heating was further evidenced by a browning of the papersheet on its coated side.

Prints were prepared from an offset master so produced usingconventional lithographic printing techniques, wherein an aqueousfountain solution is first applied over the plate surface which wets thenonimage areas leaving unwetted the oleophilic image areas. Afterapplication of the fountain solution, an ink roll coated with anoil-base printing ink was passed over the plate. Ink adhered to theimage areas exclusively of the nonimage areas, and this ink was thentransferred to a rubber blanket which transferred the ink to the finalpaper sheet. Two hundred printed copies were obtained in this manner,with good results.

It will be noted that both zinc oxide and ferric chloride were in thedielectric film coating composition set forth. Both materials wereeffective in catalyzing the degradation of the polymeric vinylidenechloride resin. Reduced amounts of such constituents may be employed,and one may be used exclusively of the other, with desen sitizing withheat being still possible. With too great a reduction in the amount ofcatalyst present, the heating period required for desensitizing becomessomewhat long. In general, to obtain a convenient desensitizing period,and when employing at least one of the catalyst materials set forthexclusively of the other, its concentration preferably should not bereduced more than about 80% from the concentration indicated. Other ironcompounds that are usable comprise such salts as ferric sulfate andferric phosphate. Large amounts of zinc oxide should be avoided, as suchtends to reduce the dielectric strength of the dielectric film. Ingeneral terms, it is preferable to maintain any zinc oxide concentrationin the coating composition at a level not exceeding about theconcentration of the polymeric vinylidene chloride resin. With ferricchloride addition greater than about 0.75% of the polymeric vinylidenechloride resin, yellow staining of the paper was noted.

The methacrylate resin, which was added for softening characteristics,and to produce toughness, should not exceed in quantity the amount ofpolymeric vinylidene resin present. With a greater proportion ofmethacrylate resin, it becomes more difiicult to degrade the dielectricfilm with heat.

Silicone additions beyond about 40% of the amount of the polymericvinylidene resin present did not show any additional benefits. With thesilicone increased to approximately the proportion of the saran resin,some intolerance of the silicone by the resin was noted. Silicone, ofcourse, may be eliminated, but the inclusion of some silicone ispreferred as a plasticizer.

Where silica aerogel is included in the dielectric film coatingcomposition, the amount of such should not exceed about 50% by weightthe amount of saran resin. This is because an excessive amount of silicatends to render the ultimate dielectric film produced somewhat crumbly,which is disadvantageous in a sheet product subjected to handling.

It will be noted that this invention contemplates a novel procedure fordesensitizing nonimage areas, whereby such areas are renderedhydrophilic and thus wettable with the usual aqueous fountain solution.The process is extremely simple, and requires no solutions orcomplicated desensitizing procedures utilizing added chemical reactants.

While particular and preferred embodiments of the invention have beendescribed, obviously variations and changes are possible withoutdeparting from the invention.

It is desired, therefore, to cover all such modifications as would beapparent to one skilled in the art, and that come within the scope ofthe appended claims.

It is claimed and desired to secure by Letters Patent:

1. A process for preparing a lithographic offset master from aconvertible plate comprising a relatively conductive sheet base and asaran resin dielectric film extending over such sheet base with suchresin selected from the group consisting of polyvinylidene chloridehomopolymers and copolymers, such film including a metalelement-containing compound operable to catalyze degradation of the filmwith the application of heat said compound being selected from the groupconsisting of zinc oxide, iron salts, and mixtures thereof, the processcomprising depositing an image-defining electrostatic charge 011 thedielectric film, developing said image-defining charge with ahydrophobic developer material which covers and fixes to the dielectricfilm in areas where the image-defining charge is located to produceareas of developed image interspersed with nonimage areas, and heatdegrading nonimage areas of the film as evidenced by discoloring thereofto render said nonimage areas hydrophilic and wettable with an aqueousfountain solution in a lithographic printing process.

2. The method of claim 1 wherein said dielectric film has silica aerogeldistributed therein with such increasing the surface area of the filmwhereby the heat degrading step rendering the film hydrophilic ispromoted.

3. In the preparation of a lithographic offset master from a plate whichincludes a saran resin dielectric film extending over a relativelyconductive sheet base, the plate having an image-defining electrostaticcharge deposited on such film, the saran resin of the film beingselected from the group consisting of polyvinylidene chloridehomopolymers and copolymers, the dielectric film containing 21 metatelement-containing compound operable to catalize degradation of the filmwith the application of heat said compound being selected from the groupconsisting of zinc oxide, iron salts, and mixtures thereof, theimprovement comprising developing the plate by applying a hydrophobicdeveloper material which covers and fixes to the dielectric film in theregion of the electrostatic charge to produce areas of developed imageinterspersed with nonimage areas, and heat degrading nonimage areas ofthe film as evidenced by discoloring thereof thereby to render saidnonimage areas hydrophilic and wettable with an aqueous fountainsolution in a lithographic printing process.

4. The process of claim 1, wherein the image charge areas are developedby distributing a heat fixable developer material over the image chargeareas which is attracted to said areas exclusively of nonimage areas onthe dielectric film, and the plate is heated simultaneously to fix thedeveloper material in said image charge areas and to degrade thedielectric film in said nonimage charge areas.

References Cited UNITED STATES PATENTS GEORGE F. LESMES, PrimaryExaminer J. C. COOPER III, Assistant Examiner

